Recently, efforts have been made to the environment in all areas. Even in vehicles, improving of fuel efficiency is a critical matter to be addressed. One of the measures to improve the fuel efficiency is reducing of the friction between a piston and a cylinder. This reduction in the friction is colligated with an enhancement in an operation performance as well as an enhancement in the fuel efficiency.
To achieve the above-mentioned friction reduction, a plateau honing method is effective. FIG. 10 is an enlarged schematic cross-sectional view of a plateau honed cylinder, and a cylinder 100 having been subjected to the plateau honing process is formed on a surface thereof with countless plateaus (Hill) 101 and valleys 102 formed between the adjacent plateaus 101, 101. A top surface 103 of the plateau 101 is low in its surface roughness thereby achieving less wear and allowing oil pooled in the valley 102 to maintain the lubrication between the piston and the top surface 103. As a result, both sliding characteristic and lubrication there-between may be realized.
As a grinding stone suitable for the plateau honing process as described above, a metal bonded grinding stone has been proposed (for example, see Patent Document 1).
In paragraph [0049] of Patent Document 1, there has been described “manufacturing conditions are that a temperature for sintering the grinding stone including barium sulfate (BaSO4) according to an embodiment is 500° C. and a molding pressure is 15 MPa. All of the grinding stones according to an illustrative embodiment have been prepared by simultaneously heating and pressing (hot press) mixed powders having been formulated”.
In the present invention, a metal bonded grinding stone material is sintered under the above-mentioned sintering conditions (500° C., 15 MPa). After sintering, although not described in Patent Document 1, the metal bonded grinding stone is obtained by stopping supplying of electric power to a heater to cool the material. In this case, the cooling rate is 5.8° C./min. A schematic cross section of the metal bonded grinding stone thus obtained is as follows.
FIG. 11 is a schematic sectional view of a related metal bonded grinding stone. In the metal bonded grinding stone 110, although it is given on the basis that cobalt (Co) grains 111, abrasive grains 112 of about 5 μm, and tungsten disulfide (WS2) grains 113 are dispersed in a metallic binder Mb, it has been found that agglomerates of about 30 μm are contained therein.
Due to insufficient dispersion of the filler which is added to improve mechanical properties, the agglomerates 115 are generated by the agglomeration of the filler cobalt grains 113 and tungsten disulfide grains 113 in a coarse crystal of the metallic binder Mb. Such agglomerates 115 are weak compared with the surrounding area.
FIG. 12 is an explanatory diagram of an action of the metal bonded grinding stone shown in FIG. 11. As a result of a grinding action having been performed with the metal bonded grinding stone 110 for a while, the agglormerates 115 are wandered from the surface thereof, and large pockets 116 having a grain size of about 30 μm are thereby generated. For this reason, the retentivity thereof becomes low thereby the quantity of grinding decreases as abrasive grains are progressively wandered and a sudden increase in abrasion thereof is generated as the agglomerates are progressively wandered. Accordingly, there is a problem that a related metal bonded grinding stone has a short life.
Also, claim 1 of Patent Document 1 recites “a super abrasive grain metal bonded grinding stone which is made by sintering and integrating soft abrasive grains containing super abrasive grains and barium sulfate which are dispersed in a sinterable metal bond containing metallic grains and glassy grains”, and claim 2 of Patent Document 1 recites “a sinterable metal bond consisting of 25 to 75 volume % metallic grains and 25 to 75 volume % glassy grains . . . ”.
Also, regarding metallic grains, it is described on paragraph [0046] of Patent Document 1 that alloy powders or mixed powders of copper (Cu) and tin (Sn) may be employed as metallic grains.
The alloy powders or mixed powders of copper (Cu) and tin (Sn) are substances that melt during sintering. As a result of reviewing the substances, the content ratio of the molten substances has been found to affect a life expectancy of the grinding stone. That is, as shown in Patent Document 1, when the content ratio of the molten substances is selected in a wide range of 25 to 75 volume %, it has been proved that variation in its life expectancy happens. Since the life expectancy of the grinding stone significantly affects productivity and production planning in a grinding process, it is necessary to stably extend the life.
Also, a table appears on paragraph [0051] of Patent Document 1. Volume ratios (%) in the grinding stone are described on lines 10 to 12 in the table as 6.2 volume % and 18.8 volume % hard abrasive grains, 12.2 to 34.7 volume % soft abrasive grains and 59.1 to 81.6 volume % binders, according to the embodiments 1 to 7. Also, it is described that the hard abrasive grains are CBN or SD (diamond) on line 3 in table 1 and the soft abrasive grains are barium sulfate (BaSO4) on line 4 in table 1.
It is described on paragraph [0031] of the same document that the preferable sizes of the super abrasive grains representative by CBN and diamond are 1 to 200 μm. Also, it is described on line 6 of paragraph [0034] in the document that the preferable grain sizes of barium sulfate are 5 to 10 μm.
It is described on paragraph [0035] of the same document that metallic grains and glassy grains are mixed as a bond (binding agent) and the sizes of the metallic grains are 1 to 50 μm. Also, it is described at the end of paragraph [00387] of the same document that the average grain sizes of the glassy grains are 3 to 5 μm.
It is described on line 2 of paragraph [0046] of the same document that metallic grains may employ alloy powders or mixed powders of copper and tin. An object of mixing metallic grains and glassy grains is described in the same document. The foregoing descriptions are listed in table 1 as follows for convenience.
TABLE 1Classifi-MixingMaterialMixing ratiocationSortspurposeExampleGrain size(volume %)AbrasiveSuper—CBN,1 to 2006.2 tograinsabrasivediamondμm18.8%grainsSoftEnhancementBarium5 to 10 μm12.2 toabrasivein a dischargesulfate34.7%grainsproperty ofcuttingpowdersMetal bondsMetallicReinforcementCopper tin1 to 50 μm59.1 to(bindinggrainsin wearalloy81.6%agent)resistanceGlassyPromotion ofGlass, silica3 to 5 μmgrainschip pocket
That is, it is described that the soft abrasive grains are mixed for the purpose of enhancing a discharge property of cutting powders, the metallic grains play a role of reinforcing the wear resistance, and the glassy grains play a role of accelerating formation of chip pockets.
Incidentally, the metal bonded grinding stone of Patent Document 1 is provided for a finishing honing process of an inner face of a cast-iron engine cylinder for a vehicle (paragraph [0030]). The Mohs hardness of cast-iron as a material to be cut and the Mohs hardness of material forming a grinding stone have been tested. This test is performed to predict what phenomenon is occurred when other substances contact-slide thereon. If the hardness thereof is known, it can be predicted which one is abraded. The Mohs hardness of cast iron is 4, the Mohs hardness of barium sulfate is 3 to 3.5, the Mohs hardness of copper and tin alloy is 3 to 4, and the Mohs hardness of glass is 5 to 7.
Generally, the process of the formation and growth of chip pockets may be explained as follows. That is, when the cast iron is ground by abrasive grains, cast iron powders (cutting powders) are generated. These cast iron powders attack and wear bond around the abrasive grains while being discharged. As a result, the chip pockets are formed and grow around the abrasive grains. According to Patent Document 1, the glassy grains as a causing material of promoting the chip pocket are harder than the cast iron (cast iron: 4, glass: 5 to 7). For this reason, the wear caused by the contact sliding of the cutting powders and glassy grains cannot be expected, and the sufficient formation and growth of the chip pockets cannot also be expected.
In the plateau honing process, valley portions and mountain portions are formed by a defective honing process, thereafter, the mountain portions only are removed during a finishing process thereby forming a hill shape. For that reason, a processing margin in the finishing process is as small as several-micrometer (μm) length. In a case where the processing margin is more than the several-micrometer length in the finishing process, even the valley portions generated by the previous rough honing process are also removed, thereby becoming a generally simple honing surface.
Here, although the super-abrasive grains corresponding to a processing margin of several-micrometer length need to be less than 10 μm, however large it may be, less than 15 μm, it is described in Patent Document 1 that the super-abrasive grain size is 1 to 200 μm. If the super-abrasive grain size is large so, since the quantity of grinding increases and the valley portions are accordingly eliminated, a preferable hill shape is not formed.
Also, regarding the grain size of barium sulfate that is used for the purpose of enhancing the discharge property of cutting powders, it is described in Patent Document 1 that the grain size of barium sulfate is 5 to 10 μm. This causes the super abrasive grains, which play a substantial grinding role, to be wandered. A detailed description will be made below. The super abrasive grain is maintained in a state of being surrounded by a metal bond as a complex. Considering this state, the exposure ratio (the quantity of protrusion) of the super abrasive grain becomes a maximum of 50% (diameter ratio, 50%=radius). In other words, no matter how strongly the super abrasive grain is maintained by a metal bond, the super abrasive grain is wandered at a point of time when the exposure ratio (the quantity of protrusion) is more than 50%.
It is described on paragraph [0022] of Patent Document 1 that when the glassy elements of a sinterable metal bond are collapsed and a chip pocket is thereby generated, the mixed barium sulfate serves to enhance the discharge property thereof due to the fluidity of the collapsed grain pieces.
Here, the grain sizes of super abrasive grain/barium sulfate/glassy grain will be described. The mark resulting from the collapse and falling of the glassy grain becomes a pocket of at least 3 to 5 μm (size of glassy grain). A number of such marks exist, as a result, the barium sulfate is wandered (it is described in Patent Document 1 that the fluidity is enhanced). However, the grain size of barium sulfate is 5 to 10 μm, when the barium sulfate is wandered, chip pockets of 5 to 10 μm are also generated. This is nearly identical in its grain size to that of the super abrasive grain performing a grinding.
That is, chip pockets having an equivalent size to that of the super abrasive grain (meanwhile, as shown in paragraph [0010] of Patent Document 1, the barium sulfate does not have the cutting property) exist. The chip pockets which are generated by an attack of cutting powders and play a role of accelerating discharging of the cutting powders are naturally generated in the surroundings of the super abrasive grains. However, a protrusion limit of the super abrasive grain is 50% of its grain size, in contrast, since the wandered marks of barium sulfate are too large as 5 to 10 μm, the super abrasive grains are easily wandered.
The wandering of the super abrasive grains of cutting blade causes the grinding ratio (life of grinding stone) to be lowered. Also, if the wandering gradually proceeds, the grinding stone performs a process in a state where the number of the super abrasive grains is small, thereby causing the efficiency of grinding (grinded volume per process hour) to be lowered.
Also, as shown in Table 1, although the mixing ratio of binder agents of 59.1 to 81.6 volume % has been calculated as the sum of metallic grains and glassy grains, the metallic grains and glassy grains are mixed in a ratio of 6:4 (embodiment of Patent Document 1). Then, the mixing ratio of glassy grains becomes about 23.6 to 32.6 volume %. If the mixing ratio of 12.2 to 34.7 volume % of barium sulfate is added to the mixing ratio of glassy grains for each embodiment, the mixing ratio becomes 41.7 to 58.3 volume %.
In such a manner, since the glassy grains and barium sulfate are wandered in a large number as described in the foregoing and the wear of grinding stone is thereby proceeded, it is concerned that the grinding ratio (life expectancy of grinding stone) is lowered.
However, since the life expectancy of grinding stone does not so affect the productivity and production planning in the grinding process, there is a need to stably increase the life expectancy thereof.